Correspondence/Rebuttal pubs.acs.org/est
Comment on “A Comprehensive Analysis of Groundwater Quality in The Barnett Shale Region” e commend the authors of “A Comprehensive Analysis of Groundwater Quality in The Barnett Shale Region”1 for compiling a large data set of 550 groundwater samples to tackle the important question of whether UOG activities in the Barnett shale have resulted in impacts on groundwater quality. The authors conclude that the data “provide a strong impetus for f urther monitoring” of possible UOG impacts on groundwater quality. We disagree with this conclusion and we find that the data set compiled by the authors, in fact, demonstrates an absence of regional groundwater impacts associated with UOG activities. When a large and comprehensive study yields negative findings it is important to clearly acknowledge these results so that future efforts can be appropriately focused on higher priority concerns.
evaluation of the authors’ data are leakage of chlorinated public water supplies and the reaction of bleach with organic matter in septic systems.7 BTEX. In the authors’ data set, the detections of individual BTEX constituents were not correlated with one another. At least one BTEX constituent was detected in 381 samples, but all four were detected in only 10 samples (3% of samples with any detection). In contrast, in Barnett Shale flowback water, all four BTEX compounds were detected in 14 of 16 samples.4 The absence of similar strong correlations in the groundwater samples, combined with the absence of a concentration vs distance relationship with gas wells in the authors’ data set, indicates that the detected BTEX constituents do not likely originate from flowback water.
THE DETECTION OF ORGANIC CONSTITUENTS DOES NOT SUGGEST IMPACTS FROM UOG ACTIVITIES The authors report that the organic chemical analyses “revealed the most striking results of this study.” They indicate that the detections of methanol (35 of 550 wells) and ethanol (240 of 550 wells); dichloromethane (122 of 550 samples), chloroform (330 of 550 samples), and trichloroethene (TCE; 14 of 550 samples); and benzene, toluene, ethylbenzene, and xylenes (BTEX; at least one BTEX constituent in 381 of 550 samples) are potentially consistent with impacts from UOG activities. We disagree with this conclusion as the chemicals and their relative concentrations are not consistent with possible UOG impacts. Methanol and Ethanol. This is the authors’ second major study of groundwater quality in the Barnett shale. In the current study, the authors do not present separate summary statistics for reference samples (defined in their prior study as samples collected outside the Barnett shale or more than 3 km from the nearest gas well).2 However, in their prior study, methanol and ethanol were each detected in approximately half of the reference samples. This suggests that causes unrelated to UOG activities are a significant source of these constituents in area groundwater. In the current study, methanol and ethanol concentrations were not correlated with each other, indicating that they originate from two distinct sources unrelated to UOG. Dichloromethane, Chloroform, and TCE. These constituents are not significantly associated with UOG activities as suggested by the authors: (i) none of these three compounds are included in the 2011 Congressional report on hydraulic fracturing3 or a list of frequently used chemicals on the national hydraulic fracturing chemical registry (www.fracfocus.org), and (ii) none of these compounds were detected in any of 16 Barnett Shale flowback water samples and they were rarely or never detected in Marcellus Shale flowback samples.4 The detection of dichloromethane in produced water reported by Maguire-Boyle and Barron5 is likely an artifact of the use of chloroform for sample extraction.5 More common sources of chloroform and dichloromethane that should be considered in
FOR INORGANICS, RELATIONSHIPS EXPECTED TO BE ASSOCIATED WITH UOG IMPACTS WERE NOT FOUND In their prior study,2 the authors identified three relationships in inorganic constituents that would be consistent with impacts from UOG activities: (i) constituent concentrations elevated relative to a historical data set, (ii) constituent concentrations that increase with proximity to the nearest gas well, and (iii) constituent concentrations higher in active extraction areas compared to nonactive/reference areas. Although these relationships were found to be present in their prior smaller study of 100 groundwater samples, the current study larger study of 550 samples does not find any of these relationships to be present. However, the authors do not interpret these negative findings in their current study to represent an absence of impacts from UOG activities. Consideration of these two studies in tandem is useful and shows that the current study does not support the prior findings. Comparison to Historical Data set. In contrast to the prior study, metals concentrations measured in this current study to not exceed the historical concentrations previously reported by the authors. This difference may be due to the fact that, for this current study, the groundwater samples were filtered to remove extraneous sediment prior to metals analysis, as were the historical groundwater samples. For the authors’ prior study, however, present-day samples were not filtered prior to metals analysis2 and the resultant concentrations were reported to exceed historical levels. The current data set indicates that the authors’ prior findings were most likely an artifact of relying on unfiltered samples. Regardless, the current data set does not support the conclusion that arsenic, selenium, and strontium concentrations in groundwater are elevated relative to the historic data set. Rather, this current finding indicates an absence of widespread UOG impacts. Concentration vs Distance Analysis. The data set in the current study shows a small but statistically significant decrease
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© 2015 American Chemical Society
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Published: December 16, 2015 496
DOI: 10.1021/acs.est.5b05067 Environ. Sci. Technol. 2016, 50, 496−497
Environmental Science & Technology
Correspondence/Rebuttal
(2) Fontenot, B. E.; Hunt, L. R.; Hildenbrand, Z. L.; Carlton, D. D., Jr.; Oka, H.; Walton, J. L.; Hopkins, D.; Osorio, A.; Bjorndal, B.; Hu, Q. H.; Schug, K. A. An evaluation of water quality in private drinking water wells near natural gas extraction sites in the barnett shale formation. Environ. Sci. Technol. 2013, 47 (17), 10032−10040. (3) Chemicals Used in Hydraulic Fracturing; United States House of Representative Committee on Energy and Commerce: Washington, DC, 2011. (4) Hayes, T. D.; Severin, B. F. Characterization of Flowback Waters from the Marcellus and Barnett; Gas Technology Institute: Des Plaines, IL, 2012. (5) Maguire-Boyle, S.; Barron, A. R. Organic Compounds in produced waters from shale gas wells. Environmental Science Processes & Impacts 2014, 16, 2237−2248. (6) Caws, A. C.; Foster, G. E. The Purity of Chloroform. J. Pharm. Pharmacol. 1957, 9, 824−833. (7) Odabasi, M. (2008). Halogenated Volatile Organic Compounds from the Use of Chlorine-Bleach-Containing Household Products. Environ. Sci. Technol. 2008, 42, 1445−1451. (8) Hayes, T. Sampling and analysis of water streams associated with the development of Marcellus shale gas, Report by Gas Technology Institute, Des Plaines, IL, for the Marcellus Shale Coalition, 2009.
in TDS with proximity to the nearest gas well. This is opposite of the relationship reported in the prior study and opposite of the relationship expected for UOG impacts. In the current paper, the authors do not report concentration vs distance relationships for metals or organics. The authors suggest that the concentration vs distance relationship for TDS is an artifact of covariance between proximity to the nearest gas well and depth of the water well sampled. However, the authors verified the statistical significance of the concentration vs distance relationship for TDS using a multivariate model that controlled for the potential covariance between distance and depth. Thus, the absence of the expected concentration vs distance relationship shows an absence of regional-scale UOG impacts to groundwater. Comparison to Reference Area. Although the authors collected a number of the current study samples for locations previously defined as nonactive/reference areas, the current study provides no comparison of concentrations in active extraction vs nonactive/reference areas.
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CONCLUSION The authors’ large data set of groundwater samples from the Barnett shale region show none of the regional-scale patterns that would be expected from impacts associated with UOG. In the absence of consistency across a number of the UOG indicator constituents (e.g., TDS, metals, BTEX), the most reasonable conclusion is that the individual patterns observed by the authors are not associated with UOG activities. Thomas McHugh Lisa Molofsky Lauren Fitzgerald John Connor* GSI Environmental Inc., 2211 Norfolk, Suite 1000, Houston, Texas 77027, United States
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AUTHOR INFORMATION
Corresponding Author
*Phone: 713-522-6033; e-mail:
[email protected]. Notes
The authors declare the following competing financial interest(s): The authors of this correspondence and their employer, GSI Environmental, are actively involved in research related to understanding the impacts of unconventional gas development on shallow groundwater and other aspects of the environment. GSI Environmental is the project lead for Reducing the Environmental Impact of Gas Shale Development: Advanced Analytical Methods for Air and Stray Gas Emissions and Produced Brine Characterization, a three-year, $4 million research project funded by the Research Partnership to Secure Energy for America (RPSEA). However, the authors and the company have no financial interest in unconventional gas development other than our involvement in this type of research.
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REFERENCES
(1) Hildenbrand, Z. L.; Carlton, D. D.; Fontenot, B.; Meik, J. M.; Walton, J.; Taylor, J.; Thacker, J.; Korlie, S.; Shelor, C. P.; Henderson, D.; Kadjo, A. F.; Roelke, C.; Hudak, P. F.; Burton, T.; Rifai, H. S.; Schug, K. A. (2015). A Comprehensive Analysis of Groundwater Quality in The Barnett Shale Region. Environ. Sci. Technol. 2015, 49 (13), 8254−8262. 497
DOI: 10.1021/acs.est.5b05067 Environ. Sci. Technol. 2016, 50, 496−497
